Regulation and Capital Adequacy - Changes in Risk-weighted Assets (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Standardized Capital Rules [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Risk-Weighted Assets, Beginning balance	$ 563,575	$ 547,910
Change in Credit RWAs	(44,748)	25,649
Change in Market RWAs	15,730	(9,984)
Change in operational risk	0
Risk-Weighted Assets, end of period	534,557	563,575
Standardized Capital Rules [Member] | Derivatives [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	(1,730)	(1,605)
Standardized Capital Rules [Member] | Commitments Guarantees and Loans [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	(16,642)	19,435
Standardized Capital Rules [Member] | Securities Financing Transactions [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	(9,426)	(496)
Standardized Capital Rules [Member] | Equity Investments [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	(11,672)	3,251
Standardized Capital Rules [Member] | Other [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	(5,278)	5,064
Standardized Capital Rules [Member] | Regulatory VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	9,389	1,151
Standardized Capital Rules [Member] | Stressed VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	7,974	2,959
Standardized Capital Rules [Member] | Incremental Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	1,077	(6,161)
Standardized Capital Rules [Member] | Comprehensive Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	255	(1,377)
Standardized Capital Rules [Member] | Specific Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	(2,965)	(6,556)
Advanced Capital [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Risk-Weighted Assets, Beginning balance	544,653	558,111
Change in Credit RWAs	29,679	(7,672)
Change in Market RWAs	15,932	(10,186)
Change in operational risk	9,362	4,400
Risk-Weighted Assets, end of period	599,626	544,653
Advanced Capital [Member] | Derivatives [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	40,309	(9,670)
Advanced Capital [Member] | Commitments Guarantees and Loans [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	2,203	(8,900)
Advanced Capital [Member] | Securities Financing Transactions [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	(3,951)	(4,425)
Advanced Capital [Member] | Equity Investments [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	(14,587)	6,738
Advanced Capital [Member] | Other [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Credit RWAs	5,705	8,585
Advanced Capital [Member] | Regulatory VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	9,389	1,151
Advanced Capital [Member] | Stressed VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	7,974	2,959
Advanced Capital [Member] | Incremental Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	1,077	(6,161)
Advanced Capital [Member] | Comprehensive Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	457	(1,579)
Advanced Capital [Member] | Specific Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Change in Market RWAs	$ (2,965)	$ (6,556)